4 Description of function
K-375 / K-376 / K-377
Operation Manual, Version E
40
Reference Substances
Name
Purity
*Commer-
cially
available
purity
% N theo-
retical
(100 %
purity)
Recom-
mended
sample
size
Recommended
titrant concen-
tration
Digestion
necessary
Ammonium
dihydrogen
phosphate
100
99.5
12.18
0.2 g
0.2 N
No
Glycine
100
99.7
18.66
0.2 g
0.2 N
Yes
Phenylala-
nine
100
99.0
8.47
0.3 g
0.2 N
Yes
Ammonium
sulfate
100
99.5
21.21
0.1 g
0.2 N
No
Tryptophan
100
99.0
13.72
0.2 g
0.2 N
Yes
Acetanilide
100
99.0
10.36
0.2 g
0.2 N
Yes
* this is only a guideline; please verify and use your specific purity of the reference substance. There-
fore, check the respective “Certificate of Analysis” which is delivered from the manufacturer of the
reference substance and create a modified reference substance according to it.
4.9 Indicator for colorimetric titration
To detect the endpoint during a colorimetric titration an indicator must be added to the boric acid. For
optimal performance the Sher mixed indicator is recommended.
The point of inflection is depending on the indicator type as well as on the added indicator amount.
The Sher indicator shows best performance in terms of endpoint detection speed and reliability.
In boric acid the color changes from green (pH >7.6) to blue (7.4 to 4.8) and finally to the gray endpoint
(pH 4.6).
The optimal ratio of the Sher indicator to boric acid is 2.5 mL per 1 L boric acid.
NOTE:
Even the slightest changes to the ratio can result in incorrect end point determination. As an alterna-
tive, methyl red/bromocresol green mix indicator can also be used. Ready-made, premixed boric acid
solutions for both indicator options can be obtained from Buchi.
4.10 Result Groups
Each result of a sample determination can be assigned to a group, e.g. the results of samples taken
from the same batch/lot, place, at the same day, etc. can be assigned to the same result group.
All results in the same group are treated the same way regarding sample printout and export of data.
